Numerous methods, which find application in different fields, of synthesizing partially crystalline Al.sub.2 O.sub.3 solid particles are known.
For example, it is known from Weinland and Stark: Chem. Ber. 59(1926), page 478, as well as from Petzold and Ulbricht: Argillaceous Earth and Argillaceous Earth Materials, Deutscher Verlag fuer Grundstoffindustrie, Leipzig, 1983, page 24, that partially crystalline and/or XRD amorphous aluminum oxides, with exceptionally large specific surface areas and high reactivities, can be obtained by the lengthy calcination of organic aluminum compounds or aluminum salts of different composition at atmospheric pressure and temperatures of more than 350.degree. C.
Because of the low availability of the starting materials and of high synthesis costs, such synthesis methods are of little applicability for the industrial-scale production of gamma Al.sub.2 O.sub.3 molded articles. Moreover, the products of the lengthy calcination of aluminum salts still contain foreign ions such as chloride, nitrate or sulfate in the oxidic solid material, which can have a very disadvantageous effect on the activity, selectivity and, accordingly, on their service life as a whole, when, for example, the product aluminum oxides are used in catalyst components.
Also, different methods have also been described for the shock-like dewatering of hydroxidic starting materials, particularly of .alpha.-Al(OH).sub.3. These methods also lead to transitional Al.sub.2 O.sub.3 forms of low crystallinity, high solid state reactivity and large specific surface area. See, for example, BRD patents U.S. Pat. No. 2,059,946 and 2,826,095 and GDR patent 250,521.
However, the decomposition products obtained with the known methods contain crystalline portions of undecomposed aluminum trihydroxides, particularly hydrargillite (also known as gibbsite) and/or portions of hydrothermally formed boehmite. As a result, the required solid state reactivity is not always attained. (See Table 2, below: solubility in NaOH and reactivity in rehydration to boehmite.)
Accordingly, in the rehydration process, there is only a small degree of conversion of the Al.sub.2 O.sub.3 particles into boehmite of fibrillar morphology, which is, however, of decisive importance for further processing, particularly for the peptization and molding of boehmitic aluminum hydroxides. Depending on the conditions of the individual technological steps of the synthesis process, Al.sub.2 O.sub.3 molded articles synthesized by known methods (based on the shock calcination of aluminum trihydroxide particles, particularly of hydrargillite), consist of Al.sub.2 O.sub.3 mixtures and, therefore, have an insufficiently developed pore structure and insufficient mechanical strength, particularly when used-as catalyst components. It is also well known that, by intensive mechanical treatment of the solid particles before and/or after the shock calcination of the Al(OH).sub.3 particles, partially crystalline, transitional aluminum oxides or their mixtures (for example, German Offenlegungsschriften 1,028,106 and 3,128,833, GDR patent 274,980, Japanese patents 80/121 914 and 82/147 437) can be obtained with high proportions of XRD amorphous solids. These oxides contain no crystalline hydroxidic portions of undecomposed hydrargillite and/or hydrothermally formed boehmite. Depending on the residence time and the temperature of the suspension, the partially crystalline, transitional aluminum oxides may be rehydrated by known methods at atmospheric pressure (as described in German Offenlegungsschriften 2,826,095 and 2,726,126 and in Japanese patent 78/144 900), as well as under hydrothermal conditions (as described in European patent publications 0 055 164 and 0 073 703 and Japanese patent 78/144 900). Boehmite-rich aluminum hydroxides some of which contain appreciable amounts of bayerite, are thereby obtained.
From a technological point of view, these prior methods require expensive equipment. For example, expensive equipment is needed when shock calcination of the hydrargillite particles is combined with a mechanical pre-treatment and/or post-treatment of the solid particles. In addition, when hydrothermal rehydration conditions are used, portions of well crystallized solid particles of a boehmitic nature are formed. These well crystallized particles are disadvantageous to further processing, particularly to peptization with inorganic or organic acids, and which are disadventageous to the goal of obtaining highly porous, solid, molded Al.sub.2 O.sub.3 articles.
German Auslegeschrift 1,200,271, German Offenlegungsschriften 2,633,599 and 3,128,833 and the Japanese patents 80/25 131 and 80/85 458 disclose a technology for producing molded Al.sub.2 O.sub.3 articles on the basis of non-ground, i.e., non-mechanically pre-activated, shock calcined aluminum trihydroxides, particularly hydrargillite. This technology is characterized by molding partially mechanically post-treated crystalline or XRD amorphous solid particles (Japanese patent 78/144 900), which molding takes place before the rehydration. As a result of the hydrothermal procedure used, this method is disadvantageous for the desired development of pores in the micropore region, particularly on the proportion of pores with a radius of r.sub.p &lt;10 nm.